Research on the Technology of Full Cylinder Concrete Bite Pile of Deep Riprap Cofferdam

In order to solve the construction problem of deep ripped-rock cofferdam occluding pile under complex geological conditions, this paper studies the occluding pile engineering of Shenzhen-Zhongshan Link Construction pipe prefabrication yard as carrier. The research contents of this paper include the formation of occlusal pile, the reasonable combination of construction equipment, the optimization of concrete mix ratio, the treatment of isolated rock and the demolition of occlusal pile, etc., forming the key technology of the construction of cofferdam concrete occlusal pile under complex geological conditions at sea. Through the construction practice, the application of this technology has obtained a good effect of water sealing, and can provide reference for subsequent similar projects.


Introduction
The borehole occluded pile has the function of water stop and support at the same time, and it does not need to be dismantled after completion of construction.It has been widely used in foundation pit and enclosure engineering in recent years.The arrangement mode of the occluding piles is the spacing arrangement between the superretarded concrete piles and the reinforced concrete piles [1][2].In the cofferdam with water stop as the main requirement and support as the auxiliary, two plain concrete piles are used in consideration of the need to remove the occluding pile after the construction of the cofferdam is completed.The strength design of the occluded pile balances the effect of water stop and the convenience of subsequent demolition.For the coastal cofferdam with large ripening depth, the construction progress and sealing effect are generally not ideal, and it is difficult to remove the coastal temporary.
The common methods of drilling biting pile include impact biting pile and rotary digging biting pile, but in the construction of deep ripped-stone layer, these two processes are easy to cause drilling deviation, borehole seepage collapse and other conditions [3][4].In order to avoid the above quality problems, the technology of plain concrete occluding pile with full protective cylinder is chosen.
In order to control the verticality of the occlusal pile and reduce the risk of hole collapse caused by gravel during pile formation, the full protective cylinder pile formation technology is selected.In the strength design of occlusal pile element concrete, the mix ratio of underwater non-diffusion concrete is adopted instead of the traditional concrete mixed with bentonite [5][6].
In this paper, a complete set of technology of plain concrete occluding pile under complex geological environment near the sea is formed by using the occluding pile as the carrier, aiming at the technical problems existing in plain concrete occluding pile, from the aspects of occluding pile formation, mechanical equipment organization, concrete mix ratio optimization, bulk gravel treatment and pre-cracking demolition of occluding pile.

Project Overview
Based on the renovation of the secondary outfitting area of the steel shell submerged tube prefabrication yard of Shenzhen-Zhongshan Channel, temporary cofferdams need to be set on the outside to form the working conditions in the dock, and the temporary cofferdams will be removed after the deep dock structure is completed.The cofferdam is about 276m long, the bottom elevation of the weir body is between -12.8m and -13.5m, the top elevation of the weir body is +6.1m, and the top elevation of the water sealing structure is +3.5m.The stone embankment center of the weir body is used, and 800-1000 stone protective surface is used on both sides.The water sealing effect will directly affect the construction period and safety in the deep dock.
The water sealing structure of the cofferdam body adopts 249 quality concrete occlusive piles, the pile diameter is 1.2 meters, and the lap joint is 30 centimeters.The water sealing structure enters the rock mass of about 0.5 meters, and its lithology is medium-weathered granite with pile length of 18.4 meters.The occlusal pile is made of underwater non-dispersed concrete, the 28d compressive strength is 2MPa, and the occlusal permeability coefficient of concrete is within 1*10-6cm/s.See figure 1 for the profile of cofferdam.The strata in the cofferdam area of Shenzhen-Zhongshan Channel are formed after many years of reef blasting, reef clearing and dredging.The stratum below the cofferdam is successively gravel, gravel, medium coarse sand, clay, and medium weathered granite.One end of the cofferdam is connected with the medium and breezy granite body, and the other end is connected with the land backfill block.Guishan Island has abundant offshore groundwater and complex formation environment.The stratigraphic division and the profile of the occluding pile into rock are as shown in figure 2.

Engineering Difficulties
(1) According to the design document, the bottom of the occluding pile needs to enter the rock layer 50cm above.The bottom of the weir body is medium-weathered granite, with more broken layers, silt and backfilled stones, which is easy to lead to insufficient depth of the occluding pile into the rock, affecting the water sealing effect and resulting in safety and quality risks.Or because the depth of the occluding pile into the rock is too large, the efficiency of pile formation is reduced and the cost is increased.
(2) The cofferdam of this project is formed by throwing stones at sea, and its protective surface is composed of a single large stone weighing 800-1000kg.The construction of occluding pile in the upper part of this cofferdam requires high vertical accuracy of pile body, and the risk of leakage after pile formation is larger.
(3) The occlusal pile in this project belongs to plain concrete pile, which needs to meet the concrete strength, the water stopping effect of occlusal pile and the convenience of subsequent demolition of occlusal pile.

Technical Principle
The construction of the interlocking pile of the deep riprap cofferdam is to cut the final hole, and pour the low strength concrete pile at certain intervals, which stops the water of the plain concrete pile in the cofferdam structure [7].

Process Flow
The occlusal pile of cofferdam water stop structure is divided into two piles: A and B, and the pile formation sequence is from A to B. Plain concrete for pile A and pile B construction shall be completed before the initial setting of pile A concrete.Cut the concrete of adjacent pile A to achieve occlusion and form pile B. As shown in figures 3-6.

Analysis of Construction Equipment
According to the design parameters and construction technical requirements, field tests and technological test piles were carried out before the construction of the full-rotating full-guard column pile, and the test pile location was selected from the original drilling position of the site.Confirm the bottom of the bearing layer in the construction area before the formal construction.In the formal construction area, the bottom of the bearing layer is determined, and the bottom criterion is formulated.The bottom determination criteria are rotary torque, rotary speed, casing pressure, casing pulling force, etc., which are determined comprehensively according to equipment conditions and drilling [8][9][10].Biting pile construction equipment, including full rotary drill, rotary dredging machine, drilling hammer drill, etc., 3 bell equipment cooperate with each other, in 3 phases of synchronous construction.
The construction technology principle of full casing full rotary drill is as follows: By using the slewing device of the full-casing full-slewing drill, the friction resistance between the steel casing and the soil layer is significantly reduced, and the friction between the steel casing and the soil layer is pressed while rotating.At the same time, the soil is excavated to the rock layer by using the impact grab bucket and impact hammer until the casing reaches the bearing layer of the pile end.After the friction between the steel casing and the soil layer is formed, the pipe is erected in the center of the drilling hole.The pile was finally injected [11][12].

Pile Machine Accuracy Control
The importance of the guide wall, the quality control of the guide wall, to prevent the deviation of the pile formation process, the pile position.Pile position lofting, pile position identification using steel bar head or wooden pile.
1. Turn the pile point back to the top surface of the guide wall as the positioning control point of the full rotary rig.Use the whole station on the site, put the positioning board of the drilling rig base in place, and then check the center and pile position of the positioning board to ensure that the two coincide consistently.
2. Use the crane to transfer the full set of pipe rotary drill to the drilling positioning plate to ensure the accurate location of the four legs at the bottom of the full rotary rig and the four positioning holes of the positioning plate.
According to the position of the pile, the point is directly turned to the top of the guide wall as the positioning control point of the full rotary rig.
3. The positioning plate of the rig base is placed in place, and then the center of the positioning plate and the pile position are checked to ensure that the two coincide.
4. Use the crawler crane to transfer the full set of pipe rotary drill to the rig positioning plate to ensure the accurate location of the four legs at the bottom of the drill and the four positioning holes of the positioning plate [13].

Verticality Control of Occlusal Pile
1.The steel plate is 4-8mm thick, the inner diameter is φ1200m m, and the length is 5-10m.Use a theodolite or wire hammer.
2. Adopt a full set of pipe rotary drill rotary device.After completing the vertical correction of the first steel casing, use the steel casing to press the oil cylinder and press the rotary synchronously under the protective cylinder.No more than 1% vertical tilt when casing down.
Before the construction of drilling occlusal pile, the sleeve shall be corrected straight on the flat ground.First, the single sleeve shall be corrected straight, and then all the sleeves configured according to the pile length are connected, and the straight deviation of the sleeve shall be controlled at 1 ‰ ~2 ‰.
Select two directions perpendicular to each other to monitor the perpendicularity of the casing above the ground.The monitoring runs through the whole process of pile, and the deviation exceeds the allowable value [14].

Construction Technology of Ultra-low Strength Underwater Undispersed Concrete
Defects existing in the original mix ratio design scheme: (1) Although the solution with bentonite and clay, low cement content and high water-binder ratio can meet the requirements of concrete strength, it has insufficient anti-dispersibility and self-compacting performance under water.When concrete specimens are formed under water, coarse aggregate will deposit, resulting in insufficient slurry at the bottom of concrete specimens, which cannot effectively enclose aggregate and expose coarse aggregate [15].
(2) Mixed with a large amount of clay and bentonite, the production process is complex, need to transform the concrete production line or increase the labor cost, not with universal applicability.
New mix ratio design idea: (1) abandon bentonite and clay, and use underwater antidispersant with better performance.
(4) Choose the stone material with high sand content, small grain size and good grading.1. Parameters of unsubmerged concrete (1) The occlusal pile has a 28d strength of 2 Mpa, and the strength should not exceed too much, so as to facilitate the subsequent demolition of the cofferdam.
(2) The penetration coefficient of the occlusal pile is 110-6 cm/s.
(3) Concrete in the tidal action of seawater, can maintain the dispersion, non-segregation of the working performance.

Low-strength underwater non-dispersed concrete ratio
Based on the performance analysis of the tests, 0.83 water cement ratio, 0.57 fly ash and 5.7% flocculant were selected as the final mix ratio.
Table 1 and table 2 show the mix ratio and performance indexes of low-strength underwater non-dispersed concrete.The concrete has excellent workability, large slump, no water seepage, no stratification, good liquidity and self-compaction [16].
Piles A and B are constructed using full rotary drill.The occlusal row pile is plain mixed with strength of 2 Mpa, with full rotary cylinder with knife head, pile B and pile A bite 30cm, and the cylinder is cut into holes.Pile B uses full rotary equipment to pull out the protective cylinder and then pour concrete for initial setting.The drilling interval of occlusal pile B is not less than 1d.The concrete mix ratio of occlusal pile is designed according to the construction requirements, and flocculant is added to prevent underwater dispersion and diffusion.The initial setting time is about 15h, and the construction interval of occlusal pile B is 24h> 15h, which can avoid the phenomenon of pile wall collapse hole or pile concrete channeling.

Rock Entry Control of Occlusal Pile
Carry out the typical construction of occlusal row pile, and determine the equipment parameters and rock samples when the complete rock surface is 50cm.In the formal construction, the pile length adopts the equipment holding parameters and the elevation double control index to avoid the rock entry deviation caused by solitary rock and large rock, and ensure that the complete rock surface is 50cm to ensure the water stop effect.
The change of tidal level and the core of loose gravel embankment will affect the occlusal row pile formation, and the full protective cylinder can avoid the influence on the hole formation.Use underwater non-dispersing concrete (add non-dispersing agent) to ensure the quality of concrete underwater pile after pouring.The rock crushing layer mainly has a certain impact on the water stop effect.After the typical construction, the infiltration test is carried out to judge the whole water stop effect.If necessary to increase the bottom grouting, if the infiltration is not treated in the acceptable range, the drainage will be carried out in the later dock.
After the steel sleeve of the full rotary drill is pressed and the slag is put into rock, the rotary excavator is used, and the rock cutting speed is appropriately increased until the cutting reaches to the bottom elevation of the design hole.

Grouting Control of Occlusal Pile and Rock Mass Lap Place
In order to ensure the grouting effect, water glass is added into the grout, and the grout is prepared according to the mix ratio of cement: water: water glass = 1:1:2:0.05.During construction, the materials needed to be weighed according to the mix ratio are added to the mixer in turn, and the mixing time is specified according to the mix ratio test.The prepared slurry is fully stirred and put into the slurry storage tank for use, and a mixer is arranged in the slurry storage tank.During the grouting process, a certain amount of grouting continuity should be ensured to prevent the slurry waiting time from being too long.If the slurry exceeds the initial setting time, the slurry should be disposed of.The key control parameters of mixing ratio, stirring time, fluidity and so on.
Special grouting pump is used in the grouting process, and the pressure must be greater than the soil pressure at the grouting depth.The grouting pressure and grouting speed are controlled during grouting, and the grouting pressure is generally controlled at 0.2-0.4MPa,and the maximum is not more than 1MPa.When the pressure is large, the speed is low, and the pressure is small, the speed is high.The principle of double control of grouting quantity and grouting pressure is adopted for the grouting termination standard at the bottom of the occluding pile hole.The grouting pressure parameter is the main parameter and the grouting quantity control is the auxiliary one.When the grouting pressure is greater than 1MPa and the grouting volume is greater than the volume of the grouting hole, the grouting can be terminated.During the grouting process, the grouting pump is operated by a special person, and the management personnel side monitor and make a good record of the grouting.

Construction Technology of Occlusal Pile Removal
The subsequent breakage of pile in the reserved blasting process of cofferdam occlusal pile, the main reasons are: (1) The vibration and shock wave generated during blasting causes damage to the nearby piles.
(2) The top of the occlusal pile produces self-weight pressure on the pile, which can easily lead to the breaking of the pile under the action of external force.
(3) Before the cofferdam occlusal pile is to crack, the backfill of both sides of the occlusal pile has been excavated for 4~5m.There is no back packing protection on both sides, and the pile is easy to break.
The cofferdam occlusal pile and water stop pile are constructed of plain concrete mixed pile with a diameter of 1.2m.150 occlusal piles are distributed within 110m to form the impermeable core wall of 1.2m thick cofferdam.The concrete strength is about 7-10Mpa, the wall top elevation is +3.3m, and the pile bottom elevation is -14.4m.In order to speed up the excavation progress of cofferdam demolition, it is necessary to use the deep pre-cracking method to carry out pre-blasting crushing from the top of pile before excavation.Due to the particularity of the construction environment, the pre-blasting should be completed from the top of the pile to the next time, with a blasting depth of 17.7m.The pre-blasting has large explosive consumption, large blasting depth, and complex construction environment.The nearest to the old and new dock piers is 57m, and the nearest to the new water pump house is 47m.
The pre-cracking blasting of the occluding pile is an internal action blasting, and the production of the concrete body is large, which is expected to reach 2.5-3kg /m3, and intensive large-aperture drilling is needed to implement the blasting.In order to reduce the impact of blasting vibration on the surrounding area, top-down time-delay blasting technology with multi-layer charge structure is needed, which has high requirements on the control of blasting charge and delay precision.

Conclusion
The full casing and full rotation construction technology can be applied to complex geological conditions such as deep rock fill layer, water-rich fault fracture zone and basement hard rock projection, and the process has good formation adaptability.
Full casing full rotary drilled pile has the advantages of good hole forming quality, strong impermeability and precise control of verticality.
The focus of the construction process of full casing and full rotation is the control of verticality, the installation quality of the steel cage and the control of sediment in the hole.
Ultra-low strength underwater non-dispersible concrete has excellent fluidity, filling and water retention, in which the flocculant can avoid the dispersion of concrete under water.
The ability of full casing to enter rock in the bedrock layer is insufficient, and the rotary drilling rig is used to drill 2-3m in advance to form holes.
In order to ensure the stability of the joint position between the occluding pile and the bedrock layer, the pressure grouting method can be used to prepare the grout according to the ratio of cement: water: water glass = 1:1:2:0.05,and the occluding pile has a good stabilization effect.

Figure 3 .
Figure 3.A Construction sequence diagram of pile B.
Overall construction sequence of interlocking row piles.

Table 1 .
Material consumption and mix proportion of 1m 3 concrete.

Table 2 .
Performance control index of underwater concrete.